1. Field of the invention
This invention relates to a driving circuit for a matrix type display device such as a matrix type liquid crystal display device.
2. Description of the prior art
Matrix type liquid crystal displays are beginning to match cathode-ray tubes in display quality as a result of a rapid advance in technology in recent years. Because of their excellent features such as thinness, light weight construction, and low power consumption, matrix type liquid crystal display devices are currently finding wide applications as display units for television receivers, visual display units for information processing apparatuses such as personal computers, and so on.
FIG. 5 shows diagrammatically one example of a conventional matrix type liquid crystal display device. In the matrix type liquid crystal display device shown in FIG. 5, thin film transistors (TFTs), which are three-terminal devices, are used as the active elements for driving picture elements. A TFT liquid crystal panel 100 comprises liquid crystal picture elements (hereinafter abbreviated as "pixels") 103 disposed in a matrix form of n rows and m columns. Each pixel 103 includes a pixel electrode 106, a counter electrode 105, and a liquid crystal layer 107 sandwiched between the two electrodes. The equivalent circuit of the pixel consists of a capacitor as shown in FIG. 5. The counter electrode 105 is usually a conductive layer disposed common to all the pixel electrodes 106. Disposed adjacent to each pixel 103 is a TFT 104, the drain electrode of which is connected to the pixel electrode 106. In the TFT liquid crystal panel 100 are disposed scanning lines 101 (the number of which is n) which are parallel to one another. To the jth scanning line 101, the gate electrodes (switching terminals) of the TFTs 104 on the jth row are connected. Signal lines 102 (the number of which is m) are disposed in such a way as to intersect perpendicularly with the scanning lines 101. To the ith signal line 102, the source electrodes (signal terminals) of the TFTs 104 on the ith column are connected.
The TFT liquid crystal panel 100 is driven by a driving circuit which includes a gate driver 200 and a source driver 300. The gate driver 200 and the source driver 300 are connected to the scanning lines 101 and the signal lines 102, respectively. A video signal is input to the source driver 300. Control signals such as scanning pulses to the gate driver 200 and sampling clock pulses to the source driver 300 are supplied from a control circuit (not shown).
FIG. 6 shows an example of display timing within one field or one frame in the matrix type liquid crystal display device of FIG. 5. The source driver 300 samples the video signal which is serially input during each horizontal scanning period initiated by a horizontal synchronizing pulse ((a) and (b) of FIG. 6). Voltages v.sub.s (j, i) (i=1, 2, . . . , m) corresponding to the amplitude of the video signal sampled during the jth horizontal scanning period jH are applied in parallel to the signal lines 102 during the (j+1)th horizontal scanning period (j+1)H ((d) of FIG. 6). On the other hand, the gate driver 200 applies a pulse to the jth scanning line during the (j+1)th horizontal scanning period (j+1)H (in FIG. 6, "g.sub.j " indicates a voltage applied to the jth scanning line 101). This energizes transistors (j, i) (i=1, 2, . . . , m) which are the TFTs 104 connected to the jth scanning line 101, thereby applying the voltage v.sub.s (J, i) to the drain electrodes of the transistors (j, i). Therefore, a voltage e(j, i) applied to the pixel 103 connected to the transistor (j, i) is given as the difference between v.sub.s (j, i) and the voltage v.sub.c applied to the counter electrode 105, i.e. v.sub.s (j, i)-v.sub.c ((h) of FIG. 6). The above described operation is hereinafter called the "writing". The writing is sequentially performed over the 1st to the nth horizontal scanning periods to complete the displaying operation for one frame or one field.
Since the pixel 103 is capacitive, the voltage written therein is held over a given period of time. The voltage applied in each field or frame has the opposite polarity from that applied in the preceding field or frame. That is, an alternating-current driving method is used in which two fields or two frames make up one complete alternating-current cycle. The use of the alternating-current driving is to prevent the pixel 103 from deteriorating due to the application of a direct current voltage.
As in a cathode-ray tube, two methods are available for displaying an image by the driving circuit on a matrix type liquid crystal display device, i.e. the interlaced scanning method and the non-interlaced scanning method.
In the non-interlaced scanning method, all the scanning lines 101 are sequentially scanned to complete one frame. In the non-interlaced scanning method, if attention is paid to a particular one of the pixels 103, writing voltage is applied to that particular pixel 103 in each frame, as shown in FIG. 7.
On the other hand, in the interlaced scanning method, one frame consists of an odd field corresponding to the odd scanning lines 101 and an even field corresponding to the even scanning lines 101, and the scanning for the odd field and that for even field are alternately performed. Interlaced scanning is used in the NTSC (National Television System Committee TV) system. As shown in FIG. 8, in the interlaced scanning method, the voltage e(2k-1, i) written into the pixels 103 of the odd columns in the odd field is held throughout the scanning period for the immediately succeeding even field ((e) of FIG. 8). Likewise, the voltage e(2k, i) written into the pixels 103 of the even columns in the even field is held throughout the scanning period for the immediately succeeding odd field ((h) of FIG. 8). Therefore, the information written in the odd field and that written in the even field are simultaneously displayed during one field period t.sub.v (t.sub.v =16.7 ms in the NTSC system). This in turn causes the problem that the image quality is deteriorated when displaying a moving picture.
When an image which can be displayed as a straight line in a still picture as shown in FIG. 9(a) is to be displayed in a moving picture which moves in the horizontal direction at a speed faster than one pixel per t.sub.v /2, the displayed images on the odd rows (scanning lines) deviate from those on the even rows ( scanning lines ) by more than one pixel as shown in FIG. 9(b), resulting in a distortion of the displayed image. Since the TFT liquid crystal panel 100 has a function of holding the written voltage for a relatively long period of time, flicker, which would be a problem with a cathode-ray tube, can be effectively improved. However, this function in turn emphasizes the after-image effect, and, therefore, causes detrimental effects when displaying a moving picture.
Such a problem does not occur in the non-interlaced scanning method. However, to display a video signal compatible to the interlaced scanning system as is used in the NTSC system, the matrix liquid crystal display device requires the provision of a frame memory or a field memory for storing sampled video signals. It further requires the provision of a high-speed A/D converter and a circuit for three-dimensional signal processing. Furthermore, since the number of the scanning lines to be scanned during one field in the non-interlaced scanning method is twice as many compared with that in the interlaced scanning method, the non-interlaced scanning system must be provided with a high-speed driving circuit including a source driver and gate driver, and with a liquid crystal panel which is capable of high-speed operation. Even if the non-interlaced scanning method is applied to a matrix type liquid crystal display device using existing techniques, however, both the driving circuit and the display device would be extremely expensive.